As has been said, 6.4:1 is incredibly low compression ratio. Lowest I have driven (not built) was 6.8:1 in a cosworth engine, but was running a lot more than 1 bar of boost, and also was never designed to be off boost, pure race. Which was a good thing, as dropping off boost meant the local 5 year old kid overtook me on his BMX! Dangerous in any road scenario as pulling out of junctions was a risky deal!
From your description, sounds like you have a very old design of engine with a reverse flow/non-crossflow style head. These will require a lot of ignition advance and the peak figures of 45-50 are about right. The best way to think about what ignition you want is to think how well the cylinder is being filled....let me explain.
The process of burning the mixture and getting it alight (notice it's being burnt...much like a piece of paper. You light it from one end and it spreads, just like in the combustion chamber) takes time. This time is obviously the ignition 'advance'. Now what affects how well something burns in the cylinder? Lets start with the basic engine design before getting onto the dynamics of an engine. The big one in this case is the compression ratio. The more you compress a mixture, the easier and quicker it is to get it to burn, hence why you hear it said that you need to 'retard' the ignition for a higher compression engine. If thats a case, the opposite must also be true (which it is). Low compression engines need more time to burn whats available. So what else effects burn? Well, the shape of the combustion chamber itself. In your engine I would expect the plug to be at one side. This means the 'flame' has to travel all the way from one side of the bore to the other, which you guessed it, takes time. This is why also big bore motors also need more advance. If it has to travel 120mm rather than 80mm, it can't be done as quick! If you notice modern 4 valve 'pent roof' style heads have the plug in the middle......shortest distance all round!! To aid all this, you also have the shape of the piston/head to promote 'swirl and squish'. This is to get all that wonderfull air fuel mixture you've tried so hard to get into the cylinder moving so it will 'carry' the flame, reducing the time it takes to get everything burnt, and making a more efficient engine.
Ok, so that was a VERY basic idea of the engine design influence on ignition timing. If you're still with me at this point, I'll move on to the engine dynamics........bet you can't wait!
Ok, so all being sat on a aftermarket ECU forum, we should know about 3D maps, and that the engine sees load and speed, and needs a timing value for these different scenarios. What has to be done is think about what the engine actually wants. Lets start at idle, we all know most engines like around 8 degrees as a base line. (I'm assuming a mild cam as in the OP). Why is this? We know the engine is in a very poor cylinder filling mode as theres a whacking great throttle plate stopping air getting in!!! Surely this means we need lots of advance to get the poor volume of air alight? Well, yes and no. Remember this is only at idle (approx 850rpm) We have loads of time to get things done. that 8 degrees of advance is actually a long time when the crank is doing 850rpm! So the burn 'time' is right, nice and long
Ok, so what about when we're cruising? At this point we still have our fairly closed throttle plate, but the RPM is now at 3krpm! A Jaffa cake to whoever guesses whats happening
Yep, I'm hoping cakes all round, now we have poor cylinder filling, but no time to get it done! So we have to 'advance' it up to get it done in time. This is why we have high advance at cruise to get good MPG...along with another reason which I'll get into in a minute! So what have we got left? Ah yes! Our favourite situation, we have the throttle buried! Now we have a nice filling cylinder as theres no throttle plate so we can drop the advance, but obviously as the rpm climbs, the advance will follow as we have less time! But what happens on boost? Well, as soon as we see boost, the cylinder is getting rammed with mixture, and the VE climbs dramaticlly, meaning it's easier to get things alight, so what do we do boys and girls? Drop the advance back
Ok, from all that, you should see a pattern forming, and it's no coincidence that when you see an ignition map in 3d view, they all follow the same traits, but now, from looking at only a well tuned ignition map you should be able to tell the engine characteristics without ever seeing it! As said, an old design 8v head will need timing in the high 40's to get a good burn, where as a modern 16v design may only need mid to low 30's peak.
I said I'd come back to the MPG thing aswell, and here it is, the last piece in this puzzle. Lean mixtures are hard to burn. What happens when summat won't burn on the fire? You add more fuel to it to get it going. Same principal. Now apply that to you ignition and fuel map
Right then, I never actually meant to type all that, but I got into the flow. Ignition timing isn't easy, but it shouldn't be a black art. Just try and think what the engine wants, as that is all we are doing wiht an ECU. Giving the engine what it wants when it wants it.
With this info you can knock up a base map to get you on a dyno, where you should aim for MBT (Minimum best timing) This is crucial guys, and will transform your engine, i promise
SO many people focus purely on fuel as it's easy to a) understand and b) measure with the wideband, but it's only one part of the big combustion process.
Obviously, as I've said, this is basic, as you can get very lost going much deeper than this, but one more thought for the guys with wild cams. Whats happening to the cylinder filling out of the cams sweet spot, and why does mapped ignition make the most gains in driveability in this case???
Thanks for reading this far, and hope it triggers a light bulb for some people.
-Gavin
(needing to re-read later as might be loads of spelling and grammar problems....!)
